Did this mob 3 times, and cleared the area just before. Even though all the fights lasted 10-15 minutes, I never had problems with respawn... it's almost like respawn became slower when this mobs is engaged.
- at 80% he splits into 3 smaller mobs, which combined hit about as bad as the initial mob. All of them must be brought to 60%. One is the original (but smaller) at 80% health, red con. The two others are at 100% health. The first one is melee resistant, the second one is immune to magic (couldn't even Tash it).
- at 60% he merges into the larger version again.
- at 40% he does the same thing, but this time they must all be brought down to 20%.
- at 20% he merges into the larger version again and can be killed. Beware, if you take too long, he starts hitting for 80k plus (50k on defensive).
If you wipe, the second round you may not be able to bring the mobs to 20% on the second split, but if you bring them to 60% and the original to 20%, they still merge again and you can finish the fight.